In conventional electrophotography processes, a photoreceptive surface is charged with a negative electrical charge, which is then exposed to an image. Because the illuminated sections (the image areas) become more conductive, the charge dissipates in the exposed areas to form a latent image. Negatively charged toner particles spread over the surface adhere to the latent image area to form a toner image. Alternatively, a photosensitive surface is uniformly charged with static electricity, and a latent image may be formed thereon by exposing image area to light. Toner particles are spread over the surface and adhere to the light-formed latent image, which has less of a negative charge than the surrounding surface, thereby forming a toner image and making the latent image visible. If required, the toner image may be transferred onto a transfer material, such as paper. The toner image may then be fixed via fixing means, such as, by heat, pressure, heat and pressure, or solvent vapor to obtain a fixed image. Such process is described, for example, in U.S. Pat. No. 2,297,691.
Typically, toners used in the development and subsequent fixing of toner images in electrophotography have been produced by melt mixing a thermoplastic resin with a coloring agent made of a dye and/or a pigment to produce a resin composition having the coloring agent uniformly dispersed therein. To obtain a toner composition having a particular particle size, the resin composition may be pulverized and/or classified to remove coarse and/or fine particles that may affect the quality of the resulting image. Optimizing the particle size distribution of the toner will allow for a high resolution image. In particular, larger particles can cause blockage while ultra fine dust particles adhere to the print head surface and are too small to have enough charge to be controllable. Thus, as higher resolution images are desired, especially high resolution color images, smaller particle sizes and narrower particle size distributions are needed. Small particles are also desirable because they typically result in improved printing speeds and lower costs per page.
The typical pulverization processes for producing these toners, while able to control the size of the toner particles to produce a high quality toner, often have certain practical limitations. For example, pulverization is a costly and inefficient process for obtaining small particle size, and puts constraints on the type of polymer that may be used, so polymers that are excellent in every other respect may be excluded because they cannot be pulverized. Additionally, a block of a resin composition in which a colorant is dispersed is required to be micro-pulverized by means of an economically usable production device. However, because the resin composition is fragile, particles having a wide range of particle sizes are easily produced when the resin composition is micro-pulverized at high speed. Additionally, such fragile material is liable to be further pulverized in a developing apparatus of a copying machine.
Furthermore, in this pulverization process, it is extremely difficult to uniformly disperse solid fine particles such as the coloring agent in a resin. Therefore, sufficient attention must be paid to the degree of dispersion to avoid potential increased fogging, a reduced image density, and decreased color mixing or transparence of the toner, depending on the degree of dispersion. Additionally, the shape and surface conditions of such toner particles, which may also greatly affect the quality of a toner image, are determined by the cleavage fractures of the resultant particles in the pulverization. Specifically, the pulverization process presents difficulties in controlling the surface conditions of the toner particles, thus when the coloring agent is exposed from the cleavage surface of fine particles of the resin composition, the quality of the developing image may be reduced.
Therefore, to overcome the problems associated with the pulverization process, it has been previously proposed to produce a chemically produced toner through polymerization, which is described, for example, in U.S. Pat. No. 4,816,366. The polymerization process is a process of producing colored polymer particles (i.e., colored resin particles) by mixing a polymerizable monomer with additive components such as a colorant, a charge control agent, and a parting agent to prepare a polymerizable monomer composition and then polymerizing the polymerizable monomer composition by suspension polymerization, emulsion polymerization, dispersion polymerization, or the like. Alternatively, chemically produced toners may also be produced by aggregating pre-formed polymers with the necessary pigment and additives. In the polymerization processes, the polymer component formed by the polymerization becomes a binder resin to directly form the colored polymer particles.
By eliminating the pulverization step, suspension polymerization or emulsion polymerization can use a softer material for toner particles that need not be as fragile. The integrity of the shape of the toner particles may be better maintained, which also prevents the coloring agent from being exposed on the surface of the toner particles. Furthermore, the classification step may optionally be omitted; thus, significant cost reduction effects such as energy savings, a reduced production time, and an improved step yield may be achieved.
However, toners produced by these polymerization processes are not without inherent limitations. For example, these limitations may include high capital requirements, resulting toners containing residual monomer or contaminated with additives, and limitations on polymer type. Specifically, with respect to the limitations on the types of polymers that may exist, typically, only polymers which can be polymerized in the presence of water may be used, thus excluding broad types of polymers. For example, polyester is a preferred resin for toner due to lower fusing temperature, better gloss, and better pigment wetting compared to styrene acrylate polymers. However, polyester is a condensation polymer which cannot be formed in an aqueous polymerization method. Polyolefin polymers similarly cannot be polymerized in an aqueous environment. With respect to residual monomers, it is difficult to completely react the polymerizable monomer in the polymerization step for forming the binder resin, and thus, an unreacted polymerizable monomer often remains in the resin. As a result, the toner may often contain residual, unreacted monomer. When the toner containing the residual, polymerizable monomer is used in an image forming apparatus, the polymerizable monomer is vaporized out of the toner by heating in a fixing step to worsen a working environment or emit offensive odor. When the content of the polymerizable monomer in the toner is high, the toner also tends to undergo blocking during its storage to aggregate or to cause an offset phenomenon or toner filming on individual members in the image forming apparatus.
Attempts to remove the polymerizable monomer have varied in their success due to the various additives that readily absorb any residual polymerizable monomer in the polymerized toner. The absorbance of the residual monomer by the additives complicates the removal of the residual monomer, as compared to removal of monomer from the binder resin alone. Even when the polymerized toner is fully washed after the polymerization, it is difficult to remove the residual polymerizable monomer adsorbed within the polymerized toner. Attempts to remove the residual polymerizable monomer by heat treatment of the polymerized toner results in aggregation of the polymerized toner.
U.S. Pat. No. 6,894,090 discloses a toner using certain types of resins, but specifically requires an organic solvent. U.S. Pat. No. 7,279,261 discloses an emulsion aggregation toner composition. Other publications discussing various aspects of toners may include U.S. Pat. Nos. 6,512,025, 5,843,614, 6,821,703, 6,521,679, 3,910,846, and 6,395,445, U.S. Patent Application Publication Nos. 20070141494, 20050271965, 20050100809, 20030232268, and 20060223934, EP Publications 170331, 1263844, 1679552, and 0246729, and PCT Application Publication WO 0201301. Toners made in some of these prior art patents and publications may be produced using a high degree of neutralization, sulfonated polyesters, high surfactant levels, and other aspects which may require additional processing steps, and may result in less than optimal toner resins. For example, use of high levels of surfactant or high degree of neutralization may decrease the environmental stability of a toner.
Accordingly, there exists a need for compositions and methods of forming high performance toner that will produce a high quality image without residual side effects.